Since 1969, work in the field of integrated optics has been expanding rapidly, until today it has become one of the most active fields of device research. For an account of work in the field one can consult such recent review articles as; P. K. Tien, Applied Optics 10, 2395 (1971), and E. T. Stepke, Electro-Optical System Design, P. 18, September, 1973.
The term "integrated optics" is used to describe devices (or systems which contain such devices) which interact with guided light waves. This requires that some component of the device have a dimension on the order of the wavelength of light, which is easily achieved in an optical fiber or thin film.
Current interest in thin film optical devices is due mainly to four factors: (a) potential physical advantages such as size, ruggedness, cost; (b) new device possibilities due to the ease of interacting guided light with planar periodic structures; (c) new device possibilities such as directional couplers which result from the ability to interact with waveguide modes; and (d) potential device performance benefits which result from the high light intensities which ought to be achievable in a small optical waveguide. The potential physical advantages are roughly akin to those realized in electronics when the transistor replaced the vacuum tube. The ultimate integrated optical device requires no external lenses or mirrors and, by the elimination of expensive mechanical fabrication techniques necessary to generate these discrete components together with their mountings, ought to result in a cheap and rugged unit.
The periodic structure is a basic component of a wide variety of integrated devices, including lasers, harmonic generators, parametric oscillators, modulators, beam deflectors, and band-stop filters. Indeed, in one configuration the periodic structure is the waveguide equivalent of a mirror.
Gratings and other optical structures with submicron dimensions are described in C. V. Shank and R. V. Schmidt, Appl. Phys. Lett. 23, 154 (1973).
Prior to this invention such optical gratings used in integrated devices were customarily made by exposing a photoresist to the interference fringe pattern of a laser.
The use of interference fringe patterns to produce fine periodic structures has serious limitations, for example, (1) although fringe patterns can be used to generate a series of parallel straight lines, any deviation from this pattern is extremely difficult, and (2) mass production using fringe pattern techniques would probably be inefficient.
The successful use of the Scanning Electron Microscope (SEM) in recent years to mass produce integrated electronic devices (see for example, G. R. Brewer "Electron and Ion Beams in Microelectronic Fabrication Process, Solid State Technology," pp. 36-39, July, 1972, and also Solid State Technology, pp. 43-47, August, 1972.) and complex accoustical structures (see for example, R. F. Henzog, High Resolution Electron Beam Fabrication, Record of the 10 Symposium on Electron, Ion and Laser Beam Techniques, Gaithersburg, Md., U.S.A. 21-23, May, 1969, pp. 107-114.) suggests its use in fabrication of fine periodic structures for optical use. "Ion Beam Micromachining of Integrated Optics Components," an article by Garvin, et al. in Applied Optics 12:3, 455 describes Corrugation in Optical Waveguides, which are said to have been made by SEM techniques.
When SEM was employed to fabricate periodic structures having significant optical properties it was discovered that pattern density imposes serious considerations upon exposing and developing of the electronic-resist, electron scattering charactertistics of the structure greatly influence the allowable beam current, and the period of the structure must be precisely controlled during fabrication.
Since SEM fabrication of optical structures can be computer controlled it is possible to fabricate much more complicated periodic structures, which might for instance consist of curved lines or of pairs of lines separated by greater distance or might consist of a grating plus a waveguide, both delineated by the position of the electron beam, and many devices on a single chip of substrate.